Thanks Craig. I wondered if Quicksync may be the culprit, but I was just dumbfounded that the difference would be that staggering between the MacPro and the MacBook Pro. Unbelievable to have a machine like the Mac Pro and have to offload to another computer to get the performance I want.

Yup. I think Larry Jordan ran test like that a few years back (and Barefeats as well).
Quicksync has a BIG advantage in accelerating H.264 encoding. The difference is not trivial at all.
This is a caveat when buying Mac Pros (including the new iMac Pro).

Interestingly the first tests I've read about do have the iMac Pro much faster with H.265 (HEVC) encoding but slower with H.264 (AVC) encoding compared to the current Quad i7 iMac.

See this Larry Jordan test from 2014. Note the HW (hardware accelerated) numbers as those are the QuickSync encodes. QuickSync is Single Pass. You can't select HW acceleration., Compressor does this automatically when you select Single Pass

"When using hardware acceleration, the Mac Pro was, on average, 29% slower than the iMac. (Speeds ranged from 2.8% faster to 73% slower.) This is due to the fact that the Mac Pro does not support hardware acceleration for compression."

"FINAL THOUGHTS

I was totally surprised by these findings. Until we start to see applications optimized to take advantage of the power of the Mac Pro, if video compression is your key task, a high-end iMac is your best choice."

AFAIK that never happened on the Mac Pro and I'm seeing similar reports about the iMac Pro.

I realize that certain computers are purpose built and the generally the MacPro will run circles around other Macs in other tasks, but oh boy. When one considers the saying 'time is money' in this regard the MacPro is too costly for such [comparably] poor results. I would at least hope there's a trick to get these Xeon processors or GPUs to get close to QuickSync performance, but it's not even in the same ball park and honesty that's kind of unacceptable when you're buying a computer like the MacPro; new or used.

I wonder if you have several machines on a network for distributed rendering/processing/transcoding, etc, if Compressor is smart enough to identify the computers with QuickSync first. Imagine a network with several Mac Pros and one or two iMacs. It would make sense just to offload to the iMacs or in my case (and sadly) my 2012 rMBP.